Based on a structure and operation method of an electrode, a conventional fluorescent lamp is classified into a HCFL (Hot Cathode Fluorescent Lamp), a CCFL (Cold Cathode Fluorescent Lamp), and an EEFL (External Electrode Fluorescent Lamp). In the HCFL and CCFL, an electrode is installed at two ends of a discharge space of the interior of a glass tube, respectively. A high voltage is supplied to the electrode for thereby generating a fluorescent light based on a discharge operation. However, the life span of the lamp is short.
In the EEFL, a glass tube is sealed, and an external electrode is installed at an outer wall of two ends of the glass tube. With this construction, the external electrode allows an electric field to be formed within the glass tube based on a capacitive coupling operation with the wall of the glass tube. The above method has a longer life span as compared to the HCFL and CCFL.
The HCFL having a tube diameter of a few centimeters has been used for a common fluorescent lamp, which needs a lot of light intensity or has been used when a power capacity is large. The CCFL and EEFL each having a tube diameter of a few millimeters has been used for a high luminance backlight or has been used when the power capacity is low.
Generally, the tube diameter of the lamp is related with light intensity based on the luminance and power capacity. As the tube diameter decreases, a higher luminance may be produced. Since the light emitting area of the fluorescent lamp is small, the light intensity is low. On the contrary, as the tube diameter increases, the luminance decreases. In this case, since the area of light emission increases, it may be well adapted to a high electric power source, which has a high light intensity. In particular, in the case of the EEFL, a small tube having a tube diameter of a few millimeters is used so as to obtain a high luminance. However, the EEFL may obtain a high luminance, but the light intensity is less. So, it is known that a high luminance cannot be obtained by simply increasing the inner diameter of the EEFL.
FIGS. 1A through 1C are views illustrating a conventional double tube fluorescent lamp. As shown in FIG. 1A, a double tube fluorescent lamp 1 comprises an outer glass lamp 10, and an inner glass lamp 11 which is formed in the interior of the outer glass tube 10. The inner and outer glass lamps are arranged at the same axis. The double tube fluorescent lamp 1 is fabricated by sealing and binding the two ends of the outer glass tube 10 and inner glass tube 11.
FIG. 1B is a cross sectional view taken in a vertical longitudinal direction of a double tube fluorescent lamp 1. Here, the cross section of a discharge shape formed between an inner wall of the outer glass wall 10 and an outer wall of the inner glass wall 1 is a circular shape, and the space formed at an inner side of the inner wall of the inner glass tube 11 is empty. Fluorescent layers 20 and 21 are coated at an inner wall of the outer glass tube 10 and an outer wall of the inner glass tube 11, respectively, with a discharge space being formed by them. A discharge gas is filled in the discharge space.
FIG. 1C is a cross sectional view taken in a vertical longitudinal direction of the double tube fluorescent lamp 1. A cylindrical empty space is formed at an inner side of the inner wall of the inner glass tube 11, and a discharge space is formed between an inner wall of the outer glass tube 10 and an outer wall of the inner glass tube 11.
Thus, the double tube fluorescent lamp generates light in such a manner that electrodes are formed outside the glass tubes which conventionally forms a double tube, and high voltage is applied to the above electrodes, and plasma is generated from the discharge gas filled in the discharge space, and then an ultraviolet ray generating from the plasma allows the fluorescent layer to excite, with the fluorescent layer being coated on the walls of the glass tube.
The Korean patent No. 10-0433193 discloses the construction of an external electrode for driving the double tube fluorescent lamp and a driving method using the same. As described in the above patent, in the driving method of the conventional double tube fluorescent lamp, an external electrode is installed between two ends of the glass tube which forms the double tube. One power source is connected with the installed external electrode, so that the plasma is generated based on the longitudinal discharge of the lamp. However, the plasma generating based on the above method is not uniform in the discharge space, and the generation of plasma is limited in the longitudinal direction. Namely, a plasma channeling phenomenon occurs. So, it is impossible to obtain a uniform discharge over the entire portions of the double tube fluorescent lamp. A high efficiency and luminance fluorescent lamp cannot be achieved in the conventional art.